Various ways are known to monitor the movement of moving objects, including objects that tend to move along a substantially predetermined path. For example, some movable barriers, such as garage doors, move along a predetermined path between opened and closed positions. By monitoring the movement of such an object, various benefits can be elicited. For example, careful monitoring of the movement of a movable barrier can support concurrent determinations regarding the likely position of the movable barrier. Such position information can be used in various ways, as is known, to facilitate both safe and efficient operation of such an apparatus.
By maintaining a count that relates to movement of an object between a first and second position (for example, by incrementing a count that correlates to revolutions of a motor output shaft, which shaft is driving movement of the object itself) a system controller can ascertain a likely position of the moving object with respect to those two positions. Unfortunately, as is known, maintaining a count that initiates at one position and continues through travel to the other position can sometimes present inaccurate results. Such inaccuracy results in part due to the tendency of the first and second positions to drift somewhat over time as a result of any number of contributing factors (including errors potentially introduced during power outages and error accrual at the terminus positions over time).
One well known system for monitoring such movement of an object between first and second positions makes use of a so-called passpoint event. The passpoint event typically comprises a signal that corresponds to a position of the moving object that is located between the first and second positions and hence is somewhat less likely to become quickly uncalibrated and then lead to inaccurate results. By resetting the count upon detecting the passpoint, overall accuracy and reliability of the count can be enhanced.
While such passpoint systems in fact provide accurate results under most operating conditions, unfortunately, even such passpoint systems are not immune to accuracy concerns under all operating conditions. For example, many movable barrier operator systems must be designed to accommodate a wide range of potential barrier travel distances (typically ranging from five to fourteen feet). A passpoint that is positioned at the seven foot mid-travel point of the fourteen foot range will function properly with a fourteen foot installation. Such a passpoint, however, would be potentially completely outside the operating range of the five foot installation. A similar problem can arise when the passpoint is set too closely to one of the terminus positions.
In general, such issues can be avoided through exercise of appropriate care during installation. For a variety of reasons, however, such care cannot always be ensured. Either through ignorance or intent, an installer can install a movable barrier operator system with the passpoint poorly chosen. As a result, the incremental count that represents movement (and hence position) of the movable barrier can be inaccurate from time to time.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are typically not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.